function twoSum(nums,target) {
    const diffs = {}; // es5 没有hashMap O(1)时间复杂度
    const len = nums.length; // 缓存数组的长度
    for (let i = 0; i < len; i++){
        const complement = target - nums[i]; // 和变成差
        if (diffs[complement]){
            return [diffs[complement],i];
        }
        
        diffs[nums[i]] = i;
    }

}